(1) Field of the Invention
This invention relates to a novel sintered apatite body comprising needle-like crystals and a novel sintered apatite-mineral fiber material composite. More particularly, this invention relates to a process for producing a novel sintered apatite body and a novel sintered composite consisting essentially of a matrix amount of an apatite and an effective amount of a reinforcing mineral fiber material, as well as the resulting sintered apatite-mineral fiber material composite. The process for sintering is characterized by baking an apatite material at a certain relatively-low temperature under pressure in the presence of water.
(2) Description of the Prior Art
The apatite materials for sintering are typically represented by hydroxyapatite, carbonate-apatite, fluoroapatite and chlororapatite. Hitherto have been proposed processes for sintering these apatites and especially hydroxyapatite at a high temperature of about 1100.degree. C. or more. These sintered hydroxyapatite products, however, had a variety of serious problems such as poor mechanical strength properties (especially, impact strength and bending strength) due to their crystalline structures, large pores or flaws, and the like. For example, refer to R. W. RICE et al., J. Am. Ceram. Soc., 63 (3-4), (1980) 129-136, and T. KIJIMA et al., J. Am. Ceram. Soc., 62, (9-10) (1979) 455-460, the description being incorporated herein by reference. These conventional sintered apatite products have been expected to be useful for broad applications such as ceramic materials, electronic materials, filter materials, bioceramics (e.g., artificial bone and tooth implants) and the like, but it can not be said that they have been successfully utilized in these uses because of their insufficient mechanical properties, etc.
An attempt to reinforce the sintered apatite with reinforcing materials such as fibers would be considered naturally by those skilled in the art. This attempt, however, has been impeded by the following fundamental problems; (i) the conventional sintered apatite body baked at such a high temperature is rather brittle and weak in mechanical strength, and thus reinforcement with such fiber materials is not so effective, and (ii) most of such reinforcing materials are markedly deteriorated in mechanical strength under the conventional baking conditions as high as 1100.degree. C. or more for sintering an apatite-reinforcing material composite.
In accordance with the conventional technical thoughts of producing a sintered apatite, it has been considered that the sintering of a pure apatite material containing no additive compound is substantially impossible at a temperature lower than 1100.degree. or 1000.degree. C. and especially lower than 800.degree. C., or that even when the apatite could be narrowly sintered at such low temperatures the physical properties of the resulting sintered apatite are inevitably decreased to uselessness. In this connection, it has been unexpectedly found that apatite can be sintered under pressurization at a temperature lower than 1000.degree. C. and preferably lower than about 800.degree. C. in the presence of water to produce a sintered apatite body having far superior mechanical properties in comparison with the conventional sintered apatite.
The present invention produces a novel sintered apatite body having superior mechanical properties and uniform porosity. Studies were also made intensive researches on a sintered apatite-reinforcing fiber material composite to further enhance its mechanical properties. In the first course of the studies, glass fibers were used and iron fibers which are excellent as reinforcing materials and readily available at low costs, but the expected increase in mechanical properties was not observed with respect to the sintered apatite composites baked at about 700.degree. C. It was then presumed that the problems were due to the deterioration of the fiber materials under oxidizing atmospheres even at about 700.degree. C. and also due to the chemical affinity of the fiber materials with apatite at such a temperature. Thus, an attempt was made to produce apatite-fiber composites wherein glass fiber or carbon fiber was used as reinforcing materials at baking temperatures lower than 400.degree. and 500.degree. C., respectively, and has unexpectedly succeeded in the production of the apatite composites having excellent properties.
The reasons for the above-mentioned phenomena have not been fully clarified yet at present. It is considered, however, that the glass fiber or iron fiber has the physical and chemical affinity with apatite at a baking temperature of about 700.degree. C. and thus the reinforcing fiber is deteriorated and also bonded to the sintered apatite firmly, whereby a sliding action between the fiber and sintered apatite is lost and the resulting composite is readily broken by the force applied thereto. On the contrary, the reinforcing fiber materials to be used in the present invention have sufficient physical affinity with apatite but are completely or substantially inert from the viewpoint of chemical affinity with apatite at the baking temperature of the present invention. Thus, it is considered that an apatite-reinforcing fiber composite having excellent mechanical properties and especially impact and bending strength properties can be unexpectedly obtained, because some sliding actions between the fiber material and sintered apatite are present in the composites according to the present invention. Incidentally, this may be rather surprising in view of the fact that, in conventional resin-fiber composites consisting of fiber materials and synthetic resins, the bonding affinity between the fibers and resins (for example by using a bonding accelerator) is desired and taken seriously, but the problems as raised above in the case of the sintered apatite-mineral fiber composites are irrelevant because the resin itself has some flexibility and the like in the case of the conventional resin composites instead of the above mentioned sliding actions.
It is an object of the invention to provide a process for producing a novel sintered apatite body and a novel sintered apatite-mineral fiber material composite.
It is another object of the invention to provide a novel sintered apatite body and a novel sintered apatite-fiber material omposite having broad ranges of applications by utilizing their excellent properties.
Other objects of the present invention will become apparent by the following description.